The present invention relates to a combined type semiconductor memory module, and in particular to a DRAM refresh method.
A list of documents referred to herein is as follows. The documents will be referred to by using a document number.
Document 1: LRS1337 Stacked Chip 32M Flash Memory and 4M SRAM Data Sheet (retrieved on Apr. 21, 2000), Internet
 less than URL:http://www.sharpsma.com/index.html greater than 
Document 2: JP-A-11-219984 (laid open in Aug. 10, 1998) (corresponding to U.S. Pat. No. 6,157,080 published in Dec. 5, 2000)
Document 3: JP-A-5-299616 (laid open in Nov. 12, 1993) (corresponding to EUROPEAN PATENT APPLICATION Publication number 566,306 laid open in Oct. 20, 1993)
Document 4: JP-A-8-305680 (laid open in Nov. 22, 1996)
Document 5: JP-A-11-204721 (laid open in Jul. 30, 1999)
Document 6: JP-A-10-11348 (laid open in Jan. 26, 1998)
In the document 1, there is described a combined type semiconductor memory module including a flash memory and an SRAM sealed integrally with a BGA (ball grid array) type package by using a stacked chip configuration. The flash memory and the SRAM share address input terminals and data input and output terminals with respect to input and output electrodes of an FBGA (Fine-pitch Ball Grid Array) type package. However, they have independent control terminals, respectively.
In the document 2, there is described a combined type semiconductor memory module including a flash memory and an SRAM integrally sealed to a BGA (ball grid array) type package by using a stacked chip. Signal pads of the flash memory are subject to face down bonding to a circuit substrate of the BGA package via solder bumps. Signal pads of the SRAM mounted on the flash memory are connected to the substrate by wire bonding.
With reference to FIG. 17 of the document 3, there is described a combined type semiconductor memory module including a flash memory chip and a DRAM chip integrally sealed to a lead frame type package. Furthermore, with reference to FIG. 1, there is described such a flash memory and a DRAM that address input terminals, data input and output terminals, and control terminals are shared for inputting and outputting with respect to input and output electrodes of the package.
In the document 4, there is described a semiconductor device. In this semiconductor device, an SRAM chip is mounted on a die pad. On the SRAM chip, a flash memory chip and a microcomputer chip connected via a bump electrode are mounted. Those chips are sealed integrally with a lead terminal type package to form the semiconductor device.
With reference to FIG. 15 of the document 5, there is described such a semiconductor device that two small-sized chips are mounted on the back of one large-sized chip via an insulation plate and those chips are integrally sealed to a lead frame type package. It is described that there are a flash memory chip, a DRAM chip, and an ASIC (Application Specific IC) as a combination of chips that can be mounted and consequently a memory embedded logic LSI is implemented by one package.
In the document 6, there is described a technique for avoiding collision between access from the outside and refresh of the DRAM by providing two DRAM blocks, storing the same data in duplicate, and staggering two DRAM blocks in refresh timing. This control is conducted by a DRAM controller. This DRAM controller issues physically independent address signals and control signals to the two DRAM blocks.
Prior to the present invention, the present inventors examined a mobile phone and a combined type memory module that is to be used for the mobile phone and that includes a flash memory and an SRAM mounted on one package. Besides the OS (operation system) of the mobile phone system, a communication program and application programs are stored in the flash memory. On the other hand, telephone numbers, an address book, and a ringing tone are stored in the SRAM. Besides, a work area which is temporarily used at the time of execution of an application is secured in the SRAM.
In order to retain data to be stored, such as the telephone numbers and the address book, power supply for retaining the data is connected to the SRAM even when power supply of the mobile phone is in the off state. For retaining the data over a long period of time, it is desirable that the data retention current of the SRAM is small. However, it is expected that with an increase of the functions added to the mobile phone (downloading of music and games) the work area used by the applications becomes large and an SRAM having a larger storage capacity is needed. In particular, enhancement in the function of recent mobile phones is remarkable. It has been found that it is gradually becoming difficult to cope with the function enhancement by using an SRAM having a larger capacity. In other words, an increase in the capacity of the SRAM has the following problem. The problem of the large capacity SRAM is that the data retention current is increased by the amount of increase of the storage capacity and in addition the data retention current is increased by increase of the gate leakage current. The reason is as follows: if advanced scaling technologies are introduced and the oxide insulation films of MOS transistors are made thinner in order to implement large capacity SRAMs, then a tunnel current flows from the gate to the substrate and the data retention current is increased thereby.
Therefore, one of objects of the present invention is to implement a memory that is larger in storage capacity and smaller in data retention current.
According to one aspect of the present invention, a flash memory, a static random access memory (SRAM), and a dynamic random access memory (DRAM) having a plurality of memory banks are incorporated into one sealing member or package. In the dynamic random access memory (DRAM), reading/writing is conducted by a command synchronized to a clock. On the sealing member, there are provided electrodes for conducting wiring to semiconductor chips and electrodes for making connections between the sealing member and the outside of the sealing member.
In order to hide refresh of the DRAM from the outside of the semiconductor device at this time, a memory controller is connected to the DRAM including two or more banks in one chip. The memory controller controls memory access to the DRAM. In the case where memory access has been conducted by the memory controller in the first interval, the first bank may be accessed. In the case where memory access has been conducted by the memory controller in the second interval, the second bank may be accessed.
A dynamic random access memory (DRAM) having a plurality of memory banks is used. In the dynamic random access memory (DRAM), reading/writing is conducted by a command synchronized to a clock. A plurality of memory banks are assigned to a first memory block and a second memory block having the memory capacity. Memory access is conducted alternately in the first interval and the second interval. In the first interval, a read/write command directed to the DRAM is executed on the first memory block, and on the second memory block, refresh operation is executed preferentially. In the second interval, a read/write command directed to the DRAM is executed on the second memory block, and on the first memory block, refresh operation may be executed preferentially.